The invention relates to the use of a peptide reagent enabling a primary Epstein-Barr virus infection to be detected, even at a very early stage, by testing in a biological sample for the possible presence of IgM antibodies that recognize the said reagent. The invention also relates to a method for detecting a primary Epstein-Barr virus infection using such a peptide reagent, as well as to a kit (detection set) enabling this method to be carried out.
The Epstein-Barr virus (abbreviation: EBV) is known to be a virus capable of infecting human epithelial and lymphoid cells. It is established that this virus is the source of infectious mononucleosis (abbreviation: IMN). Primary infection occurs most often during childhood, generally asymptomatically, and the virus thereafter remains present in the body in the latent state. It is estimated generally that more than 95% of adult humans are infected with this virus. In some countries and/or in some social groups, it is possible for primary infection not to take place until the age of adolescence or in the young adult. Among these cases of late primary infection, approximately 50% are acute infections accompanied by symptoms of infectious mononucleosis (IMN).
It is known, moreover, that, in some geographic regions, EBV is closely associated with certain cancers, in particular cancer of the nasopharynx and Burkitt""s lymphoma.
In immunosuppressed individuals, and in particular individuals who have undergone organ transplants as well as in patients infected with the HIV virus, a xe2x80x9creactivationxe2x80x9d of the virus, that is to say transition from a latency state to a lytic cycle of replication of the virus, is frequently observed. It is known that, in immunosuppressed subjects, reactivation of the virus or primary infection frequently leads to the development of various lymphomas.
The study of EBV infection and the search for methods of serological diagnosis have enabled several viral antigens to be demonstrated: the viral capsid antigens (VCA), the early antigens (EA) and the nuclear antigens (EBNA).
The classical serological diagnosis of EBV infections comprises on the one hand the test for heterophil antibodies (Paul-Bunnell-Davidsohn test), and the test for antibodies against the VCA, EA and EBNA antigens, generally by indirect immunofluorescence. These tests are difficult to carry out, and a certain proportion of false negatives and false positives are observed.
It is currently accepted that the serological profiles can be interpreted unequivocally in some cases, which are reviewed below.
Primary infection is generally followed closely by the appearance of anti-VCA antibodies, and it is accepted that the absence of anti-VCA IgG enables the conclusion to be drawn that the subject has not been infected with EBV. It is accepted, moreover, that, when a serum does not contain anti-VCA, it does not contain other anti-EBV antibodies either.
In individuals who are asymptomatic carriers, that is to say in almost the whole of the population which has been infected with the virus and in which the virus is in the latency state, both anti-VCA and anti-EBNA antibodies are found.
In the case of symptomatic primary infection (IMN), the test for heterophil antibodies (Paul-Bunnell-Davidsohn or PBD test) is generally positive. The presence of anti-VCA IgG is generally found, while anti-EBNA antibodies are absent or are present in only small amounts. The anti-VCA IgGs gradually decrease after primary infection and remain stable throughout the lifetime, while the anti-EBNA antibodies appear later, after several months, before stabilizing.
In relation to the state of knowledge about EBV infections and their diagnosis, there may be mentioned, in particular, J.-M. Seigneurinxe2x80x94Infections à Virus Epstein-Barr [Epstein-Barr Virus Infections]xe2x80x94Editions techniquesxe2x80x94Encycl. Mxc3xa9d. Chir. (Paris-France), Maladies Infectieuses, 8-071-A-10, Pxc3xa9diatrie 4-310-A-30, pp. 1-7 (1993).
It is known, moreover, that a protein called ZEBRA (or EB1 or Zta) performs a fundamental role in the regulation of latency; see, in particular, G. Miller, J. of Infectious Diseases, 161:838-844 (1990) and I. Mikaxc3xa9lian et al., J. Virol., 67:734-742 (1993).
This protein is a transcription factor which performs, in particular, an activating role in the transcription of certain genes and in its own synthesis. The ZEBRA protein is considered to be closely associated with the transition between the latency state and the lytic cycle of the virus, and the test for anti-ZEBRA antibodies has been studied as an indirect marker of a viral reactivation, in particular in HIV-seropositive immunosuppressed subjects; see, for example, I. Joab et al., J. of Infectious Diseases, 163:53-56 (1991) and V. Marxc3xa9chal et al., Res. Virol., 144:397-404 (1993).
It has now been discovered that anti-ZEBRA antibodies appear very early during primary infections, and that detection of the presence of anti-ZEBRA IgM antibodies enables primary EBV infections to be diagnosed very early. It has been discovered, in addition, that the test for IgM directed against the fragment 157-195 of the ZEBRA protein enables primary infections to be diagnosed without false negatives at a stage at which none of the traditional serological markers give 100% of positive results.
It will be recalled that the sequence 157-195 of the ZEBRA protein is a peptide corresponding to the formula (I) (SEQ ID NO: 1):
Arg Arg Thr Arg Lys Pro Gln Gln Pro Glu Ser Leu Glu Glu Cys Asp Ser Glu Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln.
By a systematic study of testing for antibodies directed against various peptides shorter than the peptide of formula (I), and whose sequence is included in the sequence represented by the formula (I), it was noted that IgMs directed against a portion of the carboxy-terminal end of the sequence of formula (I) are detected more especially in primary EBV infections. The sequence of this portion is represented by the following formula (II) (SEQ ID NO: 2):
Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln.
Hence the subject of the present invention is the use as a peptide reagent enabling a primary Epstein-Barr virus infection in a subject to be detected, and in particular detected very early, by testing for IgM antibodies that recognize the said reagent in a biological sample taken from the said subject, according to a method known per se based on the formation of at least one complex of the antigen-antibody type, characterized in that the said reagent contains a peptide recognized by at least one antibody directed against the peptide whose sequence is represented by the formula (II) given above. Bearing in mind the early stage at which the IgMs directed against the peptide of formula (II) appear, the use of the reagent according to the invention also makes it possible to confirm, with less risk of error than by using the methods known hitherto, the absence of a primary EBV infection.
It is hence possible to use, in particular, as a peptide reagent according to the invention, a peptide comprising all or part of the sequence of formula (II), including a peptide whose sequence is shorter than that of formula (II), for example a peptide of formula (III) (SEQ ID NO: 3):
Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln,
or alternatively a peptide whose sequence is longer than that of formula (II) (for example the peptide of formula (I)), or else a peptide whose sequence comprises a portion of the N-terminal end of the peptide II supplemented, on the N-terminal side, by an adjacent sequence present in the formula (I), for example a peptide of sequence (SEQ ID NO: 4):
Ser Glu Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe.
The subject of the invention is also a method for detecting a primary Epstein-Barr virus infection in a subject, characterized in that the presence of IgM antibodies capable of recognizing the peptide of formula II) (SEQ ID NO: 2):
Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln
is tested for, in a manner known per se, in a biological sample taken from the said subject.
Naturally, the presence of IgM antibodies tested for in this way enables the conclusion to be drawn that a primary EBV infection is present, and the absence of the said antibodies enables the conclusion to be drawn that a detectable primary infection is absent.
According to a particular embodiment, the method of the invention is a method in which:
a biological sample taken from the said subject is brought into contact with a peptide reagent under conditions permitting the formation of an antigen-antibody type complex between the said peptide reagent and antibodies which recognize the said reagent, if they are present in the said sample,
and the possible presence of such a complex is tested for according to a visualization method known per se, characterized in that the said peptide reagent contains a peptide recognized by at least one antibody directed against the peptide whose sequence is represented by the formula (II) (SEQ ID NO: 2):
Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln,
and in that the complexes whose presence is tested for are complexes formed between the said reagent and IgM type antibodies.
In the methods which have just been described, the peptide reagents are, in particular, the ones which have been defined above.
The biological sample is, in particular, a sample of blood serum or another biological fluid, for example cerebrospinal fluid.
The implementation of tests for detecting antibodies using an antigen is well known and hence will not be described in detail here. The antigen can be bound, by adsorption or by covalent bonding, to a solid support (well of a microtitration plate, wall of a tube, plastic bead, latex particles, and the like). When the antigen is bound to microbeads, it is possible to carry out the procedure according to immunofiltration methods, and, with latex particles, agglutination techniques can be used. It is also possible to carry out the procedure, in particular, according to the so-called xe2x80x9csandwichxe2x80x9d solid-phase techniques, or alternatively by competition with labelled antibodies specific for the antigen. In the case of the sandwich techniques, the visualization may be performed in a known manner using labelled heterologous anti-(human IgM) antibodies (animal antibodies). It is also possible to use unlabelled heterologous anti-(human IgM) antibodies and to visualize their binding to the IgM, if the latter are present, with labelled antibodies directed against the immunoglobulins of the animal species from which the heterologous antibodies used originate.
The labelling of the antibodies is performed, in particular, by coupling with an enzyme (for example a peroxidase) capable of catalysing a colour reaction. A fluorescent or luminescent label can also be used.
The formation of antigen-antibody type complexes can also be demonstrated without the use of labelled antibodies, for example using electrical measurements (capacity, impedance), in a known manner.
The subject of the invention is also a kit for detecting an Epstein-Barr virus infection, the said kit comprising a peptide reagent and means for visualizing the presence of an antigen-antibody type complex capable of being formed when the said reagent and antibodies possibly present in a biological sample to be analysed are brought into contact with one another, characterized in that, in order to detect a primary infection by the said virus:
the said peptide reagent contains a peptide recognized by at least one antibody directed against the peptide whose sequence is represented by the formula (I) (SEQ ID NO: 1):
Arg Arg Thr Arg Lys Pro Gln Gln Pro Glu Ser Leu Glu Glu Cys Asp Ser Glu Leu Glu Ile Lys Arg Tyr Lys Asn Arg Val Ala Ser Arg Lys Cys Arg Ala Lys Phe Lys Gln,
and in that the said means are means capable of visualizing the presence of antigen-antibody type complexes formed by IgM type antibodies with the said reagent.